Shield Volcanoes: The Biggest Volcanoes on Earth Look Like Gentle Hills

The largest volcanoes on Earth do not look like volcanoes. Shield volcanoes are broad, gently sloping mountains, shaped somewhat like a warrior's shield lying flat, and their volumes dwarf even large composite volcanoes. They build up from many eruptions of very fluid lava, repeated off and on for up to a million years or longer.

Mauna Loa in the background above the Kilauea caldera. Mauna Loa's summit is 13,677 feet (4,169 m) above sea level, more than 7,500 feet (2,280 m) higher than Kilauea's 4,091 feet (1,247 m) (NPS photo by David Boyle).

How big, and how gentle

Shield volcanoes are much wider than tall. Mauna Loa's base on the sea floor is about 100 miles (160 km) wide. Mount Rainier, the tallest composite volcano in the contiguous United States, is about 14,000 feet (4,270 m) tall but only about 10 miles (16 km) wide. Shield slopes are only a few percent, and flatter still near the summit, because the lava is so runny that it spreads in thin, widespread flows, erupts from the summit and from fissures on the flanks, and the summit and rift zones widen and subside. Most shield volcanoes are basalt, some mostly andesite, and they are polygenetic, meaning they erupt many times. Mauna Loa and Kilauea erupt almost entirely basalt, while the shields of Wrangell-St. Elias National Park are andesitic.

Most shield eruptions do not explode. They are effusive, usually 0 to 1 on the Volcanic Explosivity Index, in the Hawaiian style: fluid lava flows, sometimes with fire fountains. Rarely, steam-driven or magma-water eruptions occur. At Kilauea, powerful ones happened at intervals, one lasting until about 1000 CE and another from about 1500 to 1800 CE, and an explosive episode took place in the caldera in 1924.

The steam explosion at Halema'uma'u on May 22, 1924, seen from Volcano House (photographer Tai Sing Loo).

Shields in the national parks

At least 13 national parks contain shield volcanoes.

• Hawai'i Volcanoes National Park holds parts of Mauna Loa and Kilauea, two of the most active shields on Earth. They sit side by side but have separate magma systems. Mauna Loa is the largest active volcano on the planet: from its base on the sea floor it is almost 33,500 feet (10,211 m) tall, with about 19,700 feet (6,000 m) below sea level. Its summit is slightly lower than neighboring Mauna Kea's but its volume is much greater. Kilauea has erupted for most of the time since reliable written records began about 1780 CE, apart from a quiet spell from 1924 to 1952. Its East Rift Zone's Pu'u 'O'o eruption, from 1983 to 2018, was Kilauea's longest documented. More than 90% of Kilauea's surface is covered by lava flows erupted in the last 1,000 years.
• Kaloko-Honokohau National Historical Park, Hawai'i, lies on the flank of Hualalai, one of the five shield volcanoes of the island of Hawai'i.
• Wrangell-St. Elias National Park, Alaska, has eight andesitic shields. Mount Wrangell, the youngest, is considered active for a short explosive eruption reported in 1930. The older ones, such as Capital Mountain, Tanada Peak, Mount Jarvis and Mount Blackburn, have eroded and lost their shield shape.
• Bering Land Bridge National Preserve, Alaska, has five small shields in the Espenberg volcanic field, formed in the Pleistocene.
• Lava Beds National Monument, California, sits on the north flank of Medicine Lake Volcano, a large shield. Its basalt flows are about 114,000 to 1,000 years old.

Features of a shield

A schematic of a shield volcano (USGS illustration).

• Summit calderas. Large shields usually have a collapse caldera at the summit, formed as the shallow magma chamber empties by eruption or by magma moving out to a flank. They keep changing and may fill with lava, hold lava lakes or contain pit craters. In 2018 the lava lake in Halema'uma'u drained and the pit crater subsided.
• Lava lakes. Where a vent connects well to the magma supply, lava lakes can form. Mauna Loa and Kilauea have both had persistent ones.
• Rift zones. Magma moves sideways from the summit chamber to erupt through weak rift zones, which give the Hawaiian shields their elongated shape. They hold vents, fissures, pit craters, spatter cones, cinder cones and grabens.
• Lava flows. Mostly thin sheets of smooth, ropy pahoehoe or rough, rubbly 'a'a.
• Lava tubes. Lava flowing in channels under a hardened crust can leave empty tubes when it drains. In Hawai'i they sheltered Native Hawaiians and were central to some ceremonies and burials. Nahuku (Thurston Lava Tube) is in a roughly 500-year-old tube.
• Landslides are a major part of the life cycle of the Hawaiian shields.

Smooth pahoehoe over older rough 'a'a lava at Kilauea (USGS).

Hazards

Active shields produce vog, volcanic smog of sulfur dioxide gas and aerosols, which is a hazard near the volcano and downwind. Flank landslides, subsidence at calderas and pit craters, and active fumaroles can also be dangerous.

Shield volcanoes form wherever basalt, and sometimes andesite, erupts: at oceanic hot spot tracks like the Hawaiian Islands, at mid-ocean ridges, near subduction zones, in continental rift zones and in other intercontinental settings.

Sources

• National Park Service, Shield Volcanoes, Geology; rewritten in hubnx's own words.